From 12f666c1d521668911b3e0143fafb8cf8bd7952b Mon Sep 17 00:00:00 2001
From: Po Lu <luangruo@yahoo.com>
Date: Sun, 13 Mar 2022 07:22:59 +0000
Subject: [PATCH] Improve overscrolling support on Haiku

* src/haiku_support.cc (class EmacsScrollBar): New field
`real_max_value'.
(MessageReceived): Set real max value.
(MouseMoved): Get rid of magic numbers by using real max value
instead.
---
 src/haiku_support.cc | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/src/haiku_support.cc b/src/haiku_support.cc
index 98cc8e93147..5fc8d2675ee 100644
--- a/src/haiku_support.cc
+++ b/src/haiku_support.cc
@@ -1569,7 +1569,8 @@ public:
   bool can_overscroll = false;
   BPoint last_overscroll;
   int last_reported_overscroll_value;
-  int max_value;
+  int max_value, real_max_value;
+  int overscroll_start_value;
 
   EmacsScrollBar (int x, int y, int x1, int y1, bool horizontal_p) :
     BScrollBar (BRect (x, y, x1, y1), NULL, NULL, 0, 0, horizontal_p ?
@@ -1593,7 +1594,8 @@ public:
 	portion = msg->GetInt32 ("emacs:portion", 0);
 	range = msg->GetInt32 ("emacs:range", 0);
 	dragging = msg->GetInt32 ("emacs:dragging", 0);
-	proportion = (float) portion / range;
+	proportion = ((range <= 0 || portion <= 0)
+		      ? 1.0f : (float) portion / range);
 	value = msg->GetInt32 ("emacs:units", 0);
 	can_overscroll = msg->GetBool ("emacs:overscroll", false);
 
@@ -1616,6 +1618,7 @@ public:
 		SetRange (0, range - portion);
 		SetProportion (proportion);
 		max_value = range - portion;
+		real_max_value = range;
 
 		if (in_overscroll || value > max_value)
 		  value = max_value;
@@ -1634,6 +1637,7 @@ public:
 		old_value = value;
 		SetValue (value);
 		max_value = range - portion;
+		real_max_value = range;
 	      }
 	  }
       }
@@ -1879,21 +1883,20 @@ public:
 	    goto allow;
 	  }
 
-	range = max_value;
+	range = real_max_value;
 	bounds = Bounds ();
 	bounds.InsetBy (1.0, 1.0);
-	value = Value ();
+	value = overscroll_start_value;
 	trough_size = BE_RECT_HEIGHT (bounds);
 	trough_size -= BE_RECT_WIDTH (bounds) / 2;
 	if (info.double_arrows)
 	  trough_size -= BE_RECT_WIDTH (bounds) / 2;
 
-	value += ((double) range / trough_size) * diff * 2;
+	value += ((double) range / trough_size) * diff;
 
 	if (value != last_reported_overscroll_value)
 	  {
 	    last_reported_overscroll_value = value;
-	    last_overscroll = point;
 
 	    value_event.scroll_bar = this;
 	    value_event.window = Window ();
@@ -1913,6 +1916,7 @@ public:
 
 	    if (value == Value () && Proportion () < 1.0f)
 	      {
+		overscroll_start_value = value;
 		in_overscroll = true;
 		last_overscroll = point;
 		last_reported_overscroll_value = value;
-- 
2.39.5